ARM/THUMB instructions that change execution path?

Thunder and blistering!

I may have to rewrite the media instruction decoding.

I just realized that my decoding wasn't complete and the non-existent instructions are UNDEFINED.

It's one thing if the target makes an UND-exception due to bad instruction but quite different thing

if the debugger single stepping makes an UND-exception trying to find out the next address.

Thunder and blistering!

I may have to rewrite the media instruction decoding.

I just realized that my decoding wasn't complete and the non-existent instructions are UNDEFINED.

It's one thing if the target makes an UND-exception due to bad instruction but quite different thing

if the debugger single stepping makes an UND-exception trying to find out the next address.

Outch. It's good that you found out, though!

Any idea about instructions marked as UNPREDICTABLE: can it then be UNDEFINED?

In other words: UNDEFINED REQUIRES the instruction to cause UND-exception, but

MAY UNPREDICTABLE do that, or does it have to execute normally except that the result may be whatever?

(I'm not yelling even if I used caps.)

Meanwhile forging ahead forging a head.

What the heck is "unsigned saturation of a signed value" (USAT)?

Where does signed manipulation change to unsigned manipulation?

operand = Shift(R[n], shift_t, shift_n, APSR.C); // APSR.C ignored

(result, sat) = UnsignedSatQ(SInt(operand), saturate_to);

R[d] = ZeroExtend(result, 32);

(bits(N), boolean) UnsignedSatQ(integer i, integer N)

if i > 2^N - 1 then

result = 2^N - 1; saturated = TRUE;

elsif i < 0 then

result = 0; saturated = TRUE;

else

result = i; saturated = FALSE;

return (result<N-1:0>, saturated);

Is it just one-sided saturation / "rectification"?

Got a very clear answer here: Re: UPREDICTABLE instructions .

UNPREDICTABLE may be implemented as UNDEFINED.

AAARRGGHH!

I happened to encounter an instruction that looked unfamiliar. I started searching for it in the ARMv7-A/R ARM and...

...found a truckload of new instructions (fp + vector).

I had to regenerate the list all over again (updated in git). Next the spreadsheet.

I really hope I got them all this time. There seems to be 483 ARM instructions in the list, although some of them

are aliases. It's good I didn't spend much time with the Thumb instructions yet - They would have needed to be regenerated as well.

That's no fun at all. I hope you've got them all now.

When I wrote my disassembler/debugger, I recall how I went through each page of the book; actually I took all the integer instructions first.

When I was done, I had a break and worked on other things, then it struck me that I could make a floating point emulator, so I got the FFP library and added the entire FPU instruction set.

t probably took a few months before I had all instructions.

Hopefully the script will take some of the burden off.

... When you arrange the table entries and there are instructions where one has a known bit in one place and the other has a known bit in another place, it will be necessary to find out which states are valid and which are invalid. The one with invalid states should go after the one that does not have invalid states.

Eg. for instance this instruction is invalid:

     add pc,pc,pc

-So ARM decided to recycle the opcode space (because there isn't a lot of opcode space left, so this is a good thing, though writing tools become more complex).

As the above instruction contains invalid combinations of registers (basically pc is not allowed in opcode2 I believe it is; but I might be wrong - it might be only when PC is the destination).

So the instruction which takes the seat from add pc,pc,pc, should go before the add instruction.

I think it might be a good idea to modify the script for the following checks:

1: is PC in opcode2.

2: is PC the destination register.

3: is SP in opcode2.

4: is SP the destination register.

I've forgotten other rules, but the above seem to be used a few times.

Also ... some bitfield instructions are not allowed.

Rule: BFI and BFC: Start+Length must be 32 or less.

Eg. BFI r3,r6,#23,#16 is illegal

When you reach the thumb instruction set and thumb2, it's important to read about the "restrictions" for each instruction.

The 16-bit thumb instructions only allow operations on r0...r7, except for very few instructions:

ADD r7,r7,r10  /* note: destination must be the same as operand1 (the opcode actually only has room for 2 registers) */

MOV r3,r11

CMP r2,r9

The rest of them do not allow operations on r8...r15, except for ADD and SUB with SP and PC (but that's a special case. ADD and SUB #imm also allow a different range on those two registers).

I don't know everything about the instruction sets, but I'll try and write whatever I remember.

I really feel like writing a disassembler, but unfortunately, I do not have the time. :/

Have to check "add pc,pc,pc", but basically add with PC as a destination is a special instruction:

The SUBS PC, LR, #<const> instruction provides an exception return without the use of the stack. It subtracts the

immediate constant from LR, branches to the resulting address, and also copies the SPSR to the CPSR.

...

Encoding A2 ARMv4*, ARMv5T*, ARMv6*, ARMv7

<opc1>S<c> PC, <Rn>, <Rm>{, <shift>}

<opc2>S<c> PC, <Rm>{, <shift>}

<opc3>S<c> PC, <Rn>, #<const>

RRXS<c> PC, <Rn>

...

SUBS{<c>}{<q>} PC, LR, #<const> Encoding A1

<opc1>S{<c>}{<q>} PC, <Rn>, #<const> Encoding A1

<opc1>S{<c>}{<q>} PC, <Rn>, <Rm> {, <shift>} Encoding A2, deprecated

<opc2>S{<c>}{<q>} PC, #<const> Encoding A1, deprecated

<opc2>S{<c>}{<q>} PC, <Rm> {, <shift>} Encoding A2

<opc3>S{<c>}{<q>} PC, <Rn>, #<const> Encoding A2, deprecated

RRXS{<c>}{<q>} PC, <Rn> Encoding A2, deprecated

...

<opc1> The operation. <opc1> is one of ADC, ADD, AND, BIC, EOR, ORR, RSB, RSC, SBC, and SUB. ARM deprecates

the use of all of these operations except SUB.

<opc2> The operation. <opc2> is MOV or MVN. ARM deprecates the use of MVN.

<opc3> The operation. <opc3> is ASR, LSL, LSR, or ROR. ARM deprecates the use of all of these operations.

Also, I'm not sure if assembler lets instructions with lsb + length > 32 through.

Also, I'm allowing more than just user level. That drops quite some restrictions.

In the "assembly" group I got a hint that I also should do UNPREDICTABLEs, but warning about that would be nice.

That, I heard, is the convention with debuggers.

I'd like to explain a little better what I mean by the BFI and BFC:

If you come across an opcode, where the start + length is > 32, then it's not a BFI or BFC instruction.

That means that any opcodes with those values, must be handled before the BFI and BFC opcodes.

In other words: The mask+data for those opcodes must be preceding the mask+data for the BFI and BFC.

turboscrew wrote:

These need to be recognized as not UNDEFINED.

If they're all valid, then just handle them before you check for UNDEFINED.

No, not again!

It'll take another day or two to figure these out!

1 1 1 1 0 0 1 0 0 D f z n n n n d d d d 1 1 0 1 N Q M 1 m m m m     V<op><c>.F32_<Qd>,_<Qn>,_<Qm>_V<op><c>.F32_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.337

1 1 1 1 0 0 1 0 0 D f z n n n n d d d d 1 1 1 1 N Q M 0 m m m m     V<op><c>.F32_<Qd>,_<Qn>,_<Qm>_V<op><c>.F32_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.335

1 1 1 1 0 0 1 0 1 D z z n n n n d d d d 0 f 1 1 N 1 M 0 m m m m     VQD<op><c>.<dt>_<Qd>,_<Dn>,_<Dm[x]> T2/A2 A8.8.371

1 1 1 1 0 0 1 0 1 D z z n n n n d d d d 1 0 f 1 N 0 M 0 m m m m     VQD<op><c>.<dt>_<Qd>,_<Dn>,_<Dm> T1/A1 A8.8.371

1 1 1 1 0 0 1 1 0 D f f n n n n d d d d 0 0 0 1 N Q M 1 m m m m     V<op><c>_<Qd>,_<Qn>,_<Qm>_V<op><c>_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.290

1 1 1 1 0 0 1 1 0 D f z n n n n d d d d 1 1 1 0 N Q M 1 m m m m     V<op><c>.F32_<Qd>,_<Qn>,_<Qm>_V<op><c>.F32_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.281

1 1 1 1 0 0 1 1 0 D f z n n n n d d d d 1 1 1 1 N Q M 0 m m m m     VP<op><c>.F32_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.366

1 1 1 1 0 0 1 1 1 D 1 1 n n n n d d d d 1 0 z z N f M 0 m m m m     V<op><c>.8_<Dd>,_<list>,_<Dm> T1/A1 A8.8.419

1 1 1 1 0 0 1 Q 1 D z z n n n n d d d d 0 f 0 F N 1 M 0 m m m m     V<op><c>.<dt>_<Qd>,_<Qn>,_<Dm[x]>_V<op><c>.<dt>_<Dd>,_<Dn>,_<Dm[x]> T1/A1 A8.8.338

1 1 1 1 0 0 1 U 0 D z z n n n n d d d d 0 0 f 0 N Q M 0 m m m m     VH<op><c>_<Qd>,_<Qn>,_<Qm>_VH<op><c>_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.319

1 1 1 1 0 0 1 U 0 D z z n n n n d d d d 0 1 1 0 N Q M f m m m m     V<op><c>.<dt>_<Qd>,_<Qn>,_<Qm>_V<op><c>.<dt>_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.334

1 1 1 1 0 0 1 U 0 D z z n n n n d d d d 1 0 1 0 N Q M f m m m m     VP<op><c>.<dt>_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.365

1 1 1 1 0 0 1 U 1 D z z n n n n d d d d 0 f 1 0 N 1 M 0 m m m m     V<op>L<c>.<dt>_<Qd>,_<Dn>,_<Dm[x]> T2/A2 A8.8.338

1 1 1 1 0 0 1 U 1 D z z n n n n d d d d 1 0 f 0 N 0 M 0 m m m m     V<op>L<c>.<dt>_<Qd>,_<Dn>,_<Dm> T2/A2 A8.8.336

1 1 1 1 0 0 1 f 0 D z z n n n n d d d d 1 0 0 1 N Q M 0 m m m m     V<op><c>.<dt>_<Qd>,_<Qn>,_<Qm>_V<op><c>.<dt>_<Dd>,_<Dn>,_<Dm> T1/A1 A8.8.336

These need to be recognized as not UNDEFINED.

I'm very close of loosing my mind, and I'm surely getting very tired of fighting this.

I really had to do some work to find out the right 'MOV' in:

Encoding A2 ARMv4*, ARMv5T*, ARMv6*, ARMv7

<opc1>S<c> PC, <Rn>, <Rm>{, <shift>}

<opc2>S<c> PC, <Rm>{, <shift>}

<opc3>S<c> PC, <Rn>, #<const>

RRXS<c> PC, <Rn>

<opc2> The operation. <opc2> is MOV or MVN. ARM deprecates the use of MVN.

c c c c 0 0 0 1 1 0 1 S (0) (0) (0) (0) 1 1 1 1 0 0 0 0 0 0 0 0 m m m m     MOV{S}<c>_PC,_<Rm>_(=_LSL{S}<c>_PC,_<Rm>,_#0) A2 B9.3.20

(That is LSL (reg) with Rd=PC and immediate = 0).

[EDIT]

I frigging knew it (bolding is mine):

A8.8.290 VBIF, VBIT, VBSL

Encoding T1/A1 Advanced SIMD

V<op><c> <Qd>, <Qn>, <Qm>

V<op><c> <Dd>, <Dn>, <Dm>

if op == ‘00’ then SEE VEOR;

if op == ‘01’ then operation = VBitOps_VBSL;

if op == ‘10’ then operation = VBitOps_VBIT;

if op == ‘11’ then operation = VBitOps_VBIF;

and

A8.8.281 VACGE, VACGT, VACLE, VACLT

Encoding T1/A1 Advanced SIMD (UNDEFINED in integer-only variant)

V<op><c>.F32 <Qd>, <Qn>, <Qm>

V<op><c>.F32 <Dd>, <Dn>, <Dm>

Assembler syntax

where:

<op> The operation. It must be one of:

ACGE Absolute Compare Greater than or Equal, encoded as op = 0.

ACGT Absolute Compare Greater Than, encoded as op = 1.

What!

What happened to ACLE and ACLT?

What's the phone number of Sherlock Holmes?

Aha:

VACLE (Vector Absolute Compare Less Than or Equal) is a pseudo-instruction, equivalent to a VACGE instruction with

the operands reversed. Disassembly produces the VACGE instruction.

VACLT (Vector Absolute Compare Less Than) is a pseudo-instruction, equivalent to a VACGT instruction with the

operands reversed. Disassembly produces the VACGT instruction.

[/EDIT]

Yep. All instructions not matching the table are considered UNDEFINED.

It means that all those 'new' instructions must be added to the table too. (Sigh.)

Oh well, it's just a couple of hundred instructions more...

When I wrote my disassembler, there were illegal instructions, which occupied parts of legal instruction space.

In some cases, I had to make a special "illegal instruction" handling; eg. place that before the actual decoded instruction.

...

I'm quite impressed with all your work. You've absolutely done a lot in very little time!

I just took a look at the ARM_instructions.txt ...

If ignoring the condition-codes, you got 8 bits, which are almost always known.

Speed-wise it might be a real good idea to do this:

index = 0xff & (opcode >> 20);          /* isolate instruction group */

handleGroup[index](opcode);          /* jump directly to group handler */

-That means you'll shave several clock cycles off your execution time, without really sacrificing anything.

In assembly language it could of course be just a simple jump-table; r0 = opcode:

handle_group:

     ubfe r1,r0,#20,#8

     tbb [r1,lsl#1]

table:

     .4byte     MUL_AND_Group

     .4byte     MUL_AND_Group

     .4byte     MLA_EOR_Group

     .4byte     MLA_EOR_Group

     .4byte     UMAAL_SUB_Group

     .4byte     SUB_Group

     .4byte     MLS_RSB_Group

     .4byte     RSB_Group

     ...

     ...

A 256-entry table is fairly small on a RasPi

You can do this for "bits which are always known", but you can even extend it to include "bits which are often known"

Bits which are often known, could include bit 4 and perhaps bits 8...11; but it might be a good idea to wait determining what bits to include, till you have the complete table.

The above assembly code can then be declared as a function like this ...

void handle_group(uint32_t aOpcode);

and called that way; it'll indirectly jump to a C function, spending just a few clock cycles in total.

After that, you can probably focus on the low 20 bits, but in 16-bit thumb, there might be needs for modification, because 16-bit thumb does not have the 4-bit condition code field.

Unfortunately it's not so simple - this kind of things will mess it up causing "false positives":

The only bit that is either '0' or '1' (instruction specific) when the first 3 bits (27, 26, 25) after condition code field is 0 0 0, is bit 4.

If bit 4 = 0 then you can use the next bits (24, 23) but if bit 4 is '1', you have to check from bit 7 what are the next bits.

If bit 7 is '0', then next bits are 24 and 23, if bit 7 is '1', the next bits are 6 and 5.

And so on.

if the bits 27, 26 and 25 are 0 1 0, then there is only one instruction: single data transfer:

In the list it's listed like in the manual, but in the reality it's:

P = 1, pre-indexing, otherwise post-indexing or offset

U = 1 offset is added, otherwise offset is subtracted

B = 1 byte access, else word access

W = 1 writeback, else no writeback

L = 1 load, else store

It's a bit different with the media- or special LD/ST-instructions.

Sometimes all the above is not used, but are just part of opcode, sometimes B = 1 register, else immediate(?).

BTW, there will be another update to the ARM instruction list, and to the spreadsheet. I'll look into Thumbs not until I learn to do it better playing with ARM instructions first.

I think a good way might be:

check condition code

    if it's 1 1 1 1 then specials

    else normal

check bits 27 -25 (with both specials and normal)

then apply table if instruction subset contains several instructions

This way the huge amount of instructions is split into 16 subsets some of them only having a couple of instructions.

(NOTE: the floating point and vector instructions are in the special instructions - and there are lots of them.)

And if that's still far too much, I'll put them in a hash-table! That should, frigging, do it!